The present invention relates to a sensor sensitivity adjusting apparatus.
There are two conventional methods of adjusting the sensitivity of a sensor device: one using constant-current driving and the other using constant-voltage driving. The constant-current drive system controls the sensitivity of a sensor by adjusting the current that flows through the sensor. The constant-voltage drive system controls the sensor sensitivity by adjusting the voltage that is applied to the sensor.
FIG. 1 is a schematic block diagram of a conventional sensor sensitivity adjusting apparatus which adjusts the sensitivity of a sensor based on the constant-current drive system. A fixed resistor R11 and a variable resistor R12 divide the supply voltage to produce a reference voltage V1. The reference voltage V1 is applied to the plus terminal of an operational amplifier 12. The current I1 that flows in a sensor 11 is controlled by adjusting the resistance of the variable resistor R12. The current I1 that flows in the sensor 11 is defined by a fixed resistor R13, which has a stable temperature characteristic.
A detection signal from the sensor 11 is amplified by an amplifier circuit 13, and the amplified detection signal (output 14) is input to an unillustrated control circuit. The control circuit performs various controls in accordance with the detection signal from the sensor 11.
It is essential that the fixed resistor R11 and the variable resistor R12 in the conventional sensor sensitivity adjusting apparatus have approximately the same resistance ratio and approximately the same temperature characteristic. If the temperature characteristics of the two resistors R11 and R12 differ from each other, the reference voltage V1 shifts from the desired value when the temperature changes. This changes the value of the current I1, causing the sensitivity of the sensor 11 to deviate from the desired sensitivity. It is therefore troublesome to select a usable fixed resistor R11 and variable resistor R12.
Many components (five in FIG. 1), such as the fixed resistor R11, the variable resistor R12, the fixed resistor R13, the operational amplifier 12 and the amplifier circuit 13, are provided on a printed circuit board. The printed circuit board is relatively large and the attachment of these components is tiresome, thus leading to a relatively high manufacturing cost.
Accordingly, it is an object of the present invention to provide a compact sensor sensitivity adjusting apparatus having fewer components.
To achieve the above object, the present invention provides an apparatus for adjusting a sensitivity of a sensor. The apparatus includes a sensor, a current supplying circuit, a voltage applying element, and a current defining element. The sensor is activated when supplied with a predetermined drive current. The sensitivity of the sensor is changed in accordance with a value of the drive current. The current supplying circuit is connected to the sensor and supplies the drive current to the sensor based on a reference voltage. The voltage applying element has a plurality of fixed resistors, which are connected to the current supplying circuit. The voltage applying element generates the reference voltage applied to the current supplying circuit. The current defining element has a variable resistor, which is connected to the current supplying circuit and the sensor. The current defining element defines a value of a current flowing in the sensor. A sensor voltage approximately equal in a value to the reference voltage is applied to the current defining element.